Electronic circuits for driving series connected light emitting diode strings

ABSTRACT

Electronic circuits provide an error signal to control a regulated output voltage signal generated by a controllable DC-DC converter for driving one or more series connected strings of light emitting diodes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application a Divisional Application of and claims the benefit ofU.S. patent application Ser. No. 12/267,645 filed Nov. 10, 2008, whichapplication claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 60/988,520 filed Nov. 16, 2007, whichapplications are incorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

FIELD OF THE INVENTION

This invention relates generally to electronic circuits and, moreparticularly, to electronic circuits used to drive a diode load, forexample, a light emitting diode (LED) load.

BACKGROUND OF THE INVENTION

A variety of electronic circuits are used to drive diode loads and, moreparticularly, to control electrical current through strings of seriesconnected light-emitting diodes (LEDs), which, in some embodiments, forman LED display, or, more particularly, a backlight for a display, forexample, a liquid crystal display (LCD). It is known that individualLEDs have a variation in forward voltage drop from unit to unit.Therefore, the strings of series connected LEDs can have a variation inforward voltage drop.

Strings of series connected LEDs can be coupled to a common switchingregulator, e.g., a boost switching regulator, at one end of the LEDstrings, the switching regulator configured to provide a high enoughvoltage to supply each of the strings of LEDs. The other end of each ofthe strings of series connected LEDs can be coupled to a respectivecurrent sink, configured to sink a relatively constant current througheach of the strings of series connected LEDs.

It will be appreciated that the voltage generated by the commonswitching regulator must be a high enough voltage to supply the oneseries connected string of LEDs having the greatest total voltage drop,plus an overhead voltage needed by the respective current sink. In otherwords, if four series connected strings of LEDs have voltage drops of30V, 30V, 30V, and 31 volts, and each respective current sink requiresat least one volt in order to operate, then the common boost switchingregulator must supply at least 32 volts.

While it is possible to provide a fixed voltage switching regulator thatcan supply enough voltage for all possible series strings of LEDs, sucha switching regulator would generate unnecessarily high powerdissipation when driving strings of series connected LEDs having lessvoltage drop. Therefore, in some LED driver circuits, the voltage dropsthrough each of the strings of series connected LEDs are sensed (forexample, by a so-called “minimum select circuit”) to select a lowestvoltage appearing at the end of one of the strings of series connectedLEDs and the common switching regulator is controlled to generate anoutput voltage only high enough to drive the series connected LED stringhaving the lowest voltage (i.e., the highest voltage drop). One suchminimum select circuit is described, for example, in U.S. Pat. No.6,822,403.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an electroniccircuit for driving a plurality of series connected light emitting diodestrings with a controllable DC-DC converter includes a plurality ofcurrent regulators, each having a respective input node and a respectiveoutput node, the input node or the output node coupled to an end of arespective one of the plurality of series connected light emitting diodestrings. Each current regulator is configured to pass a respectivepredetermined current through the respective one of the plurality ofseries connected light emitting diode strings to which it is coupled.The electronic circuit also includes a multi-input error amplifierhaving a plurality of input nodes and an output node. Each one of theplurality of input nodes is coupled to the input node or the output nodeof a respective one of the plurality of current regulators. Themulti-input error amplifier is configured to generate an error signal atthe output node of the error amplifier.

In accordance with another aspect of the present invention, anelectronic circuit for driving a plurality of series connected lightemitting diode strings with a controllable DC-DC converter includes aplurality of current regulators, each having a respective input node anda respective output node, the input node or the output node coupled toan end of a respective one of the plurality of series connected lightemitting diode strings. Each current regulator is configured to pass arespective predetermined current through the respective one of theplurality of series connected light emitting diode strings to which itis coupled. The electronic circuit also includes a plurality of erroramplifiers, each having a respective input node and a respective outputnode. Each one of the plurality of input nodes of the plurality of erroramplifiers is coupled to the input node or the output node of arespective one of the plurality of current regulators. The output nodesof the plurality of error amplifiers are coupled to a junction node. Theplurality of error amplifiers is configured to generate an error signalat the junction node.

In accordance with another aspect of the present invention, anelectronic circuit for driving a plurality of series connected lightemitting diode strings with a controllable DC-DC converter includes aplurality of current regulators, each having a respective input node anda respective output node, the input node or the output node coupled toan end of a respective one of the plurality of series connected lightemitting diode strings. Each current regulator is configured to pass arespective predetermined current through the respective one of theplurality of series connected light emitting diode strings to which itis coupled. The electronic circuit also includes a plurality ofswitches, each having a respective input node, a respective output node,and a respective control node. Each one of the input nodes of theplurality of switches is coupled to the input node or the output node ofa respective one of the plurality of current regulators. The outputnodes of the plurality of switches are coupled together resulting is acomposite signal. The electronic circuit also includes a digital channelselect circuit coupled to the control nodes of the plurality of switchesand configured to close each one of the plurality of switchessequentially and periodically. The electronic circuit also includes anerror amplifier having an input node and an output node. The input nodeof the error amplifier is coupled to receive the composite signal. Theerror amplifier is configured to generate an error signal at the outputnode of the error amplifier.

In accordance with another aspect of the present invention, anelectronic circuit for driving a plurality of series connected lightemitting diode strings with a controllable DC-DC converter includes aplurality of field effect transistors (FETs), each FET having arespective drain, source, and gate. Each FET is configured to pass apredetermined current from the respective drain to the respectivesource. The electronic circuit also includes a plurality of resistors,each having respective first and second ends, each resistor coupled atthe first end to a respective source of one of the plurality of FETs,forming a respective current sense node. The drain of each FET or thesecond end of each resistor is coupled to an end of a respective one ofthe plurality of series connected light emitting diode strings. Theelectronic circuit also includes a plurality of amplifiers, eachamplifier having a respective input node coupled to a respective currentsense node, and each amplifier having a respective output node coupledto a respective gate of a respective FET. Each one of the plurality ofamplifiers is configured to generate a respective control voltage signalat the respective output node indicative of a control of the respectiveFET for the respective FET to pass the predetermined current from therespective drain to the respective source. The electronic circuit alsoincludes a maximum select circuit having a plurality of input nodescoupled to receive the control voltage signals from the plurality ofamplifiers and having an output node. The maximum select circuit isconfigured to select a largest one of the control voltage signals and togenerate a signal representative of the largest one of the controlvoltage signals at the output node. The electronic circuit also includesan error amplifier having an input node and an output node. The inputnode of the error amplifier is coupled to the output node of the maximumselect circuit. The error amplifier is configured to generate an errorsignal at the output node of the error amplifier.

In accordance with another aspect of the present invention, anelectronic circuit for driving a plurality of series connected lightemitting diode strings with a controllable DC-DC converter includes aplurality of current regulators, each having a respective input node anda respective output node, the input node or the output node coupled toan end of a respective one of the plurality of series connected lightemitting diode strings. Each current regulator is configured to pass arespective predetermined current through the respective one of theplurality of series connected light emitting diode strings to which itis coupled. The electronic circuit also includes a plurality ofswitches, each having a respective input node, a respective output node,and a respective control node. Each one of the input nodes of theplurality of switches is coupled to the input node or the output node ofa respective one of the plurality of current regulators. The outputnodes of the plurality of switches are coupled together resulting is acomposite signal. The electronic circuit also includes a comparatorcoupled to receive the composite signal and configured to generate acomparison signal. The electronic circuit also includes a digitalchannel select circuit coupled to receive the comparison signal andcoupled to the control nodes of the plurality of switches and configuredto close each one of the plurality of switches sequentially for a timeperiod responsive to the comparison signal. The electronic circuit alsoincludes an error amplifier having an input node and an output node. Theinput node of the error amplifier is coupled to receive the compositesignal. The error amplifier is configured to generate an error signal atthe output node of the error amplifier.

In accordance with another aspect of the present invention, a method ofdriving a plurality of series connected light emitting diode stringswith a controllable DC-DC converter includes attempting to pass arespective predetermined current through each one of the plurality ofseries connected light emitting diode strings, resulting in a respectivevoltage appearing at an end of each one of the plurality of seriesconnected light emitting diode strings. The method also includes summingeach one of the voltages to generate an error signal to control theDC-DC converter.

In accordance with another aspect of the present invention, a method ofdriving a plurality of series connected light emitting diode stringswith a controllable DC-DC converter includes attempting to pass arespective predetermined current through each one of the plurality ofseries connected light emitting diode strings, resulting in a respectivevoltage appearing at an end of each one of the plurality of seriesconnected light emitting diode strings. The method also includesgenerating respective intermediate signals representative each one ofthe voltages, and summing the intermediate signals to generate an errorsignal to control the DC-DC converter.

In accordance with another aspect of the present invention, a method ofdriving a plurality of series connected light emitting diode stringswith a controllable DC-DC converter includes attempting to pass arespective predetermined current through each one of the plurality ofseries connected light emitting diode strings, resulting in a respectivevoltage appearing at an end of each one of the plurality of seriesconnected light emitting diode strings. The method also includessampling each one of the voltages sequentially and periodically togenerate voltage samples, and summing the voltage samples to generate anerror signal to control the DC-DC converter.

In accordance with another aspect of the present invention, a method ofdriving a plurality of series connected light emitting diode stringswith a controllable DC-DC converter includes attempting to pass arespective predetermined current through each one of the plurality ofseries connected light emitting diode strings with a respective feedbackcurrent control circuit, resulting in a respective voltage appearing atan end of each one of the plurality of series connected light emittingdiode strings. A control node of the feedback circuit generates acontrol voltage that changes in a direction opposite to a change of therespective voltage. The method also includes detecting a largest one ofthe control voltages, and generating an error signal representative ofthe largest one of the control voltages to control the DC-DC converter.

In accordance with another aspect of the present invention, a method ofdriving a plurality of series connected light emitting diode stringswith a controllable DC-DC converter includes attempting to pass arespective predetermined current through each one of the plurality ofseries connected light emitting diode strings, resulting in a respectivevoltage appearing at an end of each one of the plurality of seriesconnected light emitting diode strings. The method also includessampling each one of the voltages sequentially to generate voltagesamples, and comparing each one of the voltage samples to a thresholdsignal to generate a comparison signal. Each one of the voltage sampleshas a time period responsive to the comparison signal. The method alsoincludes summing the voltage samples to generate an error signal tocontrol the DC-DC converter.

The above-described circuits and method provide a controllable DC-DCconverter to drive a plurality of series connected light emitting diodestrings. The controllable DC-DC converter is controlled in such a way asto provide just enough voltage so as to minimize the power dissipationin the a plurality of series connected light emitting diode stringswhile not being overly affected if one of the a plurality of seriesconnected light emitting diode strings becomes open circuited.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention, as well as the invention itselfmay be more fully understood from the following detailed description ofthe drawings, in which:

FIG. 1 is a schematic diagram of an electronic circuit for driving adiode load, the electronic circuit having a controllable DC-DCconverter, current regulators, and a multiple-input error amplifierconfigured to provide an error signal to control an output voltagegenerated by the controllable DC-DC converter;

FIG. 2 is a schematic diagram of a switching regulator circuit that canbe used as the controllable DC-DC converter of FIG. 1;

FIG. 3 is a schematic diagram of an exemplary amplifier that can be usedas the multiple-input error amplifier of FIG. 1;

FIG. 4 is a schematic diagram of another electronic circuit for drivinga diode load, the electronic circuit having a controllable DC-DCconverter, current regulators, and a plurality of error amplifiersconfigured to provide an error signal to control an output voltagegenerated by the controllable DC-DC converter;

FIG. 5 is a schematic diagram of another electronic circuit for drivinga diode load, the electronic circuit having a controllable DC-DCconverter, current regulators, and a plurality of switches coupled to anerror amplifier configured to provide an error signal to control anoutput voltage generated by the controllable DC-DC converter;

FIG. 6 is a schematic diagram of another electronic circuit for drivinga diode load, the electronic circuit having a controllable DC-DCconverter, current regulators including FETs and associated currentsense circuits, a maximum select circuit, and an error amplifierconfigured to provide an error signal to control an output voltagegenerated by the controllable DC-DC converter;

FIG. 7 is a schematic diagram of an exemplary maximum select circuitthat can be used as the maximum select circuit of FIG. 6;

FIG. 8 is a schematic diagram of another exemplary maximum selectcircuit that can be used as the maximum select circuit of FIG. 6; and

FIG. 9 is a schematic diagram of another electronic circuit for drivinga diode load, the electronic circuit having a controllable DC-DCconverter, current regulators, a comparator, and a plurality of switchescoupled to an error amplifier configured to provide an error signal tocontrol an output voltage generated by the controllable DC-DC converter.

DETAILED DESCRIPTION OF THE INVENTION

Before describing the present invention, some introductory concepts andterminology are explained. As used herein, the term “boost switchingregulator” is used to describe a known type of switching regulator thatprovides an output voltage higher than an input voltage to the boostswitching regulator. While a certain particular circuit topology ofboost switching regulator is shown herein, it should be understood thatboost switching regulators have a variety of circuit configurations. Asused herein, the term “buck switching regulator” is used to describe aknown type of switching regulator that provides an output voltage lowerthan an input voltage to the buck switching regulator. It should beunderstood that there are still other forms of switching regulatorsother than a boost switching regulator and other than a buck switchingregulator, and this invention is not limited to any one type.

DC-DC converters are described herein. The described DC-DC converterscan be any form of switching regulator, including, but not limited to,the above-described boost and buck switching regulators.

As used herein, the term “current regulator” is used to describe acircuit or a circuit component that can regulate a current passingthrough the circuit or circuit component to a predetermined, i.e.,regulated, current. A current regulator can be a “current sink,” whichcan input a regulated current, or a “current source,” which can output aregulated current. A current regulator has a “current node” at which acurrent is output in the case of a current source, or at which a currentis input in the case of a current sink.

As used herein, the term “current sense circuit” is used to describe acircuit that can sense a regulated current passing through a circuit. Insome particular arrangements, the current sense circuit provides avoltage output proportional to a sensed current.

Referring to FIG. 1, an exemplary electronic circuit 10 includes acontrollable DC-DC converter 12 coupled to series connected diodestrings 14, 16, 18, which, in some arrangements, are series connectedlight emitting diode (LED) strings as may form an LED display or abacklight for a display, for example, a liquid crystal display (LCD). Asdescribed above, in some arrangements, the controllable DC-DC converter12 is a switching regulator, one type of which is described more fullybelow in conjunction with FIG. 2. The series connected LED strings 14-18are coupled to respective current regulators 20, 22, 24, here shown tobe current sinks. Each one of the current regulators 20, 22, 24 has arespective voltage sense node 20 a-24 a.

Since the series connected LED strings 14-18 can each generate adifferent voltage drop, the voltages appearing at the voltage sensenodes 20 a-24 a can be different. It will also be recognized that atleast a predetermined minimum voltage must be present at each of thevoltage sense nodes in order for the current regulators 20, 22, 24 tofunction properly, i.e., to sink the desired current for which they aredesigned.

A multi-input error amplifier 32 is coupled to receive voltage signals26, 28, 30 corresponding to voltages appearing at the voltage sensenodes 20 a-24 a, respectively, at an inverting input node. Themulti-input error amplifier 32 is also coupled to receive a referencevoltage signal 31, for example, 0.5 volts, at a non-inverting inputnode. The multi-input error amplifier 32 is configured to generate anerror signal 34, which is related to an opposite of an arithmetic meanof the voltage signals 26-30. In some particular arrangements, themulti-input error amplifier 32 has inputs comprised of metal oxidesemiconductor (MOS) transistors, as shown below in FIG. 3. In somearrangements, the error amplifier 32 is a transconductance amplifier,which provides a current-type output.

The circuit 10 can include a capacitor 36. The capacitor 36 can becomprised of an output capacitance of the multi-input error amplifier 32in parallel with an input capacitance of an error node 12 b of thecontrollable DC-DC converter 12. However, in some other arrangements,the capacitor 36 can include another capacitor as well. In oneparticular arrangement, the capacitor 36 has a value of about onehundred picofarads. The capacitor 36 can provide a loop filter and canhave a value selected to stabilize a feedback control loop.

The controllable DC-DC converter 12 is coupled to receive the errorsignal 34 at the error node 12 b of the controllable DC-DC converter 12.The controllable DC-DC converter 12 is also coupled to receive a powersupply voltage, Vps, at an input node 12 c and to generate a regulatedoutput voltage 38 at an output node 12 a in response to the error signal34. In some arrangements, the controllable DC-DC converter 12 is a boostswitching regulator and the controllable DC-DC converter 12 is coupledto receive the power supply voltage, Vps, at the input node 12 c and togenerate a relatively higher regulated output voltage 38 at the outputnode 12 a.

With this arrangement, the controllable DC-DC converter 12 is controlledby an arithmetic mean of the voltage signals 26, 28, 30. Thus, a voltagesignal 26, 28, 30 that would be too low to provide proper operation ofan associated one of the current regulators 20, 22, 24 will result in anincrease in the error signal 34, tending to raise the output voltage 38of the controllable DC-DC converter 12.

It should be appreciated that the regulated output voltage 38 has aparticular desired value. Specifically, the particular desired value ofthe regulated output voltage 38 is that which achieves a high enoughvoltage at all of the current regulators 20, 22, 24 so that they can alloperate properly to regulate current as desired. In addition, theparticular desired value of the regulated output voltage 38 is thatwhich is as low as possible so that the one or more of the currentregulators that receive the lowest voltage(s) (i.e., the greatestvoltage drop across the associated series connected LED strings 14, 16,18) have just enough voltage to properly operate. With this particulardesired value of the regulated output voltage 38, a low power isexpended in the current regulators 22, 24, 26, resulting in high powerefficiency while properly illuminating the LEDs.

In some particular arrangements, the desired value of regulated voltage38 can include a voltage margin (e.g., one volt). In other words, insome arrangements, the particular desired value of the regulated outputvoltage 38 is that which is as low as possible so that the one or moreof the current regulators that receive the lowest voltage(s) have justenough voltage to properly operate, plus the voltage margin. Still, alow power consumption results.

The above described error signal 34, which is the arithmetic mean of thevoltage signals 26, 28, 30, approximately achieves the particulardesired value of the regulated output voltage 38.

Certain elements of the circuit 10 can be within a single integratedcircuit. For example, in some arrangements, the current regulators 20,22, 24, the multi-input amplifier 32, the capacitor 36, and someinternal elements of the controllable DC-DC converter 12 (described morefully below in conjunction with FIG. 2) can be within the singleintegrated circuit.

In some alternate arrangements, the multi-input error amplifier 32 isreplaced by a multi-input comparator, which either has hysteresis, orwhich is periodically clocked at which time it makes a comparison.

In some alternate embodiments, the current regulators 20-24, which areshown to be coupled to the bottom (cathode) ends of the series connectedLED strings 14-18, respectively, can instead be at to top (anode) endsof the series connected LED strings 14-18, respectively. In theseembodiments, the input nodes 20 a-24 a are coupled to receive theregulated output voltage 38, and output nodes 20 b-24 b are coupled tothe anode ends of the series connected LED strings 14-18, respectively.Furthermore, in these embodiments, the inverting inputs of the erroramplifier 32 are coupled to the output nodes 20 b-24 b, which become thevoltage sense nodes in place of the input nodes 20 a-24 a, and thenon-inverting input of the error amplifier 32 is coupled to receive adifferent reference voltage.

The circuit 10 has advantages over the prior art. For example, thecircuit 10 avoids the necessity for the above-described minimum selectcircuit, which can result in less integrated circuit die area.

Referring now to FIG. 2, in which like elements of FIG. 1 are shownhaving like reference designations, the controllable DC-DC converter 12can include a portion 14 that can be within the above-describedintegrated circuit, and a portion 16 that can be external to but coupledto the integrated circuit.

The portion 14 can include a pulse width modulation (PWM) controller 18coupled to receive the error signal 34 from the multiple-input erroramplifier 32 of FIG. 1. The PWM controller 18 is configured to generatea PWM signal 20. A control current passing element, for example, a FET22, is coupled to receive the PWM signal 20 at a gate node and toreceive a pulsed current signal 24 at a drain node.

The portion 16 can include an input capacitor 26 coupled between thepower supply voltage, Vps, received at the node 12 c and a groundvoltage. An inductor 28 can have an input node 28 a also coupled toreceive the input voltage, Vps, and an output node 28 b coupled to thedrain node of the FET 22. A diode 30 can have an anode coupled to theoutput node 28 b of the inductor 28 and a cathode coupled to the outputnode 12 a, at which the regulated output voltage, Vreg, is generated. Anoutput capacitor 32 can be coupled between the output node 12 a and theground voltage.

Referring now to FIG. 3, a multi-input error amplifier 50 can be thesame as or similar to the multi-input error amplifier 32 of FIG. 1. Themulti-input error amplifier 50 can include a non-inverting node 54 aassociated with a metal oxide semiconductor (MOS) field effecttransistor (FET). The multi-input error amplifier 50 can also include aplurality of inverting input nodes, here shown as three inverting inputnodes 56 a, 58 a, 60 a, associated with MOSFETs 56, 58, 60,respectively. One of ordinary skill in the art will understand that,with this particular arrangement, the gain of the multi-input erroramplifier 50 will be proportional to the number of inverting inputs thatare used. Therefore, as described above, the gain of the multi-inputerror amplifier 50 is proportional to an arithmetic mean of signalsapplied to the three inverting input ports 56 a, 58 a, 60 a.

Referring now to FIG. 4, in which like elements of FIG. 1 are shownhaving like reference designations, an exemplary electronic circuit 70includes error amplifiers 78, 80, 82. The error amplifier 78 is coupledto receive a voltage signal 72 at an inverting input node and configuredto generate an error signal 78 a, the error amplifier 80 is coupled toreceive a voltage signal 74 at an inverting input node and configured togenerate an error signal 80 a, and the error amplifier 82 is coupled toreceive a voltage signal 76 at an inverting input node and configured togenerate an error signal 82 a. The voltage signals 72, 74, 76 can be thesame as or similar to the voltage signals 26, 28, 30, respectively, ofFIG. 1. The error amplifiers 78, 80, 82 are also coupled to receive areference voltage 77, for example, 0.5 volts, at their non-invertinginput nodes. The error signals 78 a, 80 a, 82 a sum to generate an errorsignal 84 in a particular way described more fully below. In somearrangements, the error amplifiers 78, 80, 82 are transconductanceamplifiers, which provide current-type outputs.

The circuit 70 can include a capacitor 86 coupled to the output nodes ofthe error amplifiers 78, 80, 82. The capacitor 86 can be comprised of aparallel combination of output capacitances of the error amplifiers 78,80, 82 in parallel with the input capacitance of the error node 12 b ofthe controllable DC-DC converter 12. However, in some otherarrangements, the capacitor 86 can include another capacitor as well. Inone particular arrangement, the capacitor 86 has a value of about onehundred picofarads. The capacitor 86 can provide a loop filter and canhave a value selected to stabilize a feedback control loop.

In one particular arrangement, the error signals 78 a, 80 a, 82 a sum ina particular way to generate the error signal 84. In particular, theoutput stages (not shown) of the amplifiers 78, 80, 82 can be configuredto provide a larger current in one direction than in the otherdirection. In other words, the output stages of the amplifiers 78, 80,82 can source more current than they can sink, or vice versa. With thisarrangement, for example, if the amplifiers 78, 80, 82 can source morecurrent than they can sink, and if the error signal 84 is lower involtage than one of the amplifiers 78, 80, 82 attempts to generate, theamplifier attempting to drive the voltage of the error signal 84 highercan at least partially override other ones of the amplifiers 78, 80, 82,which are attempting drive the voltage of the error signal 84 lower. Forthis particular example, since the amplifiers 78, 80, 82 are invertingamplifiers, the amplifier attempting to drive the error signal 84 higheris associated with a current regulator 20, 22, 24, which has a voltagesense node 20 a, 22 a, 24 a, at which a lowest voltage occurs.

One of ordinary skill in the art will recognize that an amplifier withasymmetrical output current drive ability is fashioned by way ofasymmetrically sized output transistors in an output stage of theamplifier.

The controllable DC-DC converter 12 is coupled to receive the errorsignal 84 at the error node 12 b of the controllable DC-DC converter 12.The controllable DC-DC converter 12 is also coupled to receive the powersupply voltage, Vps, at the input node 12 c and to generate a regulatedoutput voltage 88 at the output node 12 a in response to the errorsignal 84. It should be recognized that the regulated output voltage 88can be the same as or similar to the regulated output voltage 38 ofFIG. 1. However, since the error signal 84 is generated in a differentway than the error signal 34 of FIG. 1, the regulated output voltage 88need not be exactly the same as the regulated output voltage 38.

With this arrangement, the controllable DC-DC converter 12 is controlledpredominantly by one or more of the amplifiers 78, 80, 82, which iscoupled to one or more of the current regulators 20, 22, 24 having thelowest voltage. However, other ones of the amplifiers 78, 80, 82 alsocontribute to the error signal 84, but with less influence. Thus, avoltage signal 72, 74, 76 that would otherwise be too low to provideproper operation of an associated one of the current regulators 20, 22,24 will result in an increase in the error signal 84, tending to raisethe regulated output voltage 88 of the controllable DC-DC converter 12.

A particular desired value of the regulated output voltage 38 isdescribed above in conjunction with FIG. 1, and the same particulardesired value applies in the same way to the regulated output voltage88. The above described error signal 84, which is dominated by one ormore of the signals 78 a, 80 a, 82 a, which are representative of arespective lowest one or more of the voltage signals 72, 74, 76,approximately achieves the particular desired value of the regulatedoutput voltage 88.

Certain elements of the circuit 70 can be within a single integratedcircuit. For example, in some arrangements, the current regulators 20,22, 24, the amplifiers 78, 80, 82, the capacitor 86, and some internalelements of the controllable DC-DC converter 12 (described more fullyabove in conjunction with FIG. 2) can be within the single integratedcircuit.

In some alternate arrangements, the error amplifiers 78, 80, 82 can bereplaced by comparators, for which the outputs can be combined with anOR gate.

In some alternate embodiments, the current regulators 20-24, which areshown to be coupled to the bottom (cathode) ends of the series connectedLED strings 14-18, respectively, can instead be at to top (anode) endsof the series connected LED strings 14-18, respectively. In theseembodiments, the input nodes 20 a-24 a are coupled to receive theregulated output voltage 38, and output nodes 20 b-24 b are coupled tothe anode ends of the series connected LED strings 14-18, respectively.Furthermore, in these embodiments, the inverting inputs of the erroramplifiers 78-82 are coupled to the output nodes 20 b-24 b, which becomethe voltage sense nodes in place of the input nodes 20 a-24 a, and thenon-inverting inputs of the error amplifiers 78-82 are coupled toreceive a different reference voltage.

The circuit 70 has advantages over the prior art. For example, thecircuit 70 avoids the necessity for the above-described minimum selectcircuit, which can result in less integrated circuit die area.Furthermore, for embodiments in which the error amplifiers 78, 80, 82have asymmetrical output drive capabilities as described above, a loopgain of the circuit 70 tends to change (e.g., drop) as more of thecurrent regulators 20, 22, 24 come into regulation, i.e., receivesufficiently high voltage signals 72, 74, 76. The lower gain of the loopresults in a drop of the error signal 84 as soon as any of the currentregulators 20, 22, 24 begin to regulate. For embodiments in which thecontrollable DC-DC converter 12 is a boost switching regulator(described more fully below in conjunction with FIG. 2), this tends toimprove feedback loop stability and reduce overshoot and ringing thatmight occur during any voltage step, for example, at turn on of thecircuit 70.

As yet another advantage, for some arrangements similar to the circuit70, one or more of the series connected LED string 14, 16, 18 canreceive a different regulated voltage, for example, from a differentrespective one of more DC-DC converters (not shown). This arrangement isadvantageous for circuits that require that a respective one or more ofthe current regulator 20, 22, 24 regulate to a different current. Forexample, if the two current regulators 20, 22 and associated two seriesconnected LED strings 14, 16 were passing twenty milliamps and the onecurrent regulator 24 and associated series connected LED string 18 werepassing one hundred milliamps, then the series connected LED string 18would require a higher regulated voltage than the regulated voltage 88.Examples where different currents are required include RGB(red-green-blue) applications where each series connected LED string hasdifferent colored LEDs or provides a backlight for different coloredLEDs. Another example is a circuit for flash applications where someseries connected LED strings would be for backlighting and other seriesconnected LED string would be for a flash application.

Referring now to FIG. 5, in which like elements of FIG. 1 are shownhaving like reference designations, an exemplary electronic circuit 90includes switches 98, 100, 102, coupled to receive voltage signals 92,94, 96, respectively. The voltage signals 92, 94, 96 can be the same asor similar to the voltage signals 26, 28, 30, respectively, of FIG. 1.The switches are also coupled to receive a control signal 112 generatedby a digital channel select module 110, which causes the switches 98,100, 102 to close sequentially and periodically, one at a time, forsubstantially equal periods, resulting in sequential and periodicvoltage signals 104, 106, 108, which directly combine into a compositesignal 114. In one particular arrangement, the control signal 112 has afrequency of about one hundred kilohertz.

An error amplifier 116 is coupled to receive the composite signal 114 atan inverting input node, to receive a reference voltage 115, forexample, 0.5 volts, at a non-inverting input node, and configured togenerate an error signal 118. In some arrangements, the error amplifier116 is a transconductance amplifier, which provides a current-typeoutput.

The circuit 90 can include a capacitor 120 coupled to the output node ofthe error amplifier 116. The capacitor 120 can be comprised of aparallel combination of output capacitance of the error amplifier 116 inparallel with the input capacitance of the error node 12 b of thecontrollable DC-DC converter 12. However, in some other arrangements,the capacitor 120 can include another capacitor as well. In oneparticular arrangement, the capacitor 120 has a value of about onehundred picofarads. The capacitor 120 can provide a loop filter and canhave a value selected to stabilize a feedback control loop.

The output stage (not shown) of the amplifier 116 can be configured toprovide a larger current in one direction than in the other direction.In other words, the output stage of the amplifier 116 can source morecurrent than it can sink, or vice versa. With this arrangement, forexample, if the amplifier 116 can source more current than it can sink,and if the error signal 118 is lower in voltage than one of the voltagesignals 104, 106, 108 attempts to generate during its associated timeperiods within the composite signal 114, the amplifier 118 responds bydriving the error signal 118 higher, giving dominance to the lowest oneor more of the voltage signals 104, 106, 108.

An amplifier with asymmetrical output current drive capability can befashioned by way of asymmetrically sized output transistors in an outputstage of the amplifier.

The controllable DC-DC converter 12 is coupled to receive the errorsignal 118 at the error node 12 b of the controllable DC-DC converter12. The controllable DC-DC converter 12 is also coupled to receive thepower supply voltage, Vps, at the input node 12 c and to generate aregulated output voltage 122 at the output node 12 a in response to theerror signal 118. It should be recognized that the regulated outputvoltage 122 can be the same as or similar to the regulated outputvoltage 38 of FIG. 1. However, since the error signal 118 is generatedin a different way than the error signal 34 of FIG. 1, the regulatedoutput voltage 122 need not be exactly the same as the regulated outputvoltage 38.

With this arrangement, the controllable DC-DC converter 12 is primarilycontrolled by a lowest one or more of the voltage signals 104, 106, 108and other ones of the voltage signals 104, 106, 108 can have lessinfluence. Thus, a voltage signal 92, 94, 96 that would otherwise be toolow to provide proper operation of an associated one of the currentregulators 20, 22, 24 will result in an increase in the error signal118, tending to raise the regulated output voltage 122 of thecontrollable DC-DC converter 12.

With this arrangement, the controllable DC-DC converter 12 is controlledpredominantly by one or more of the voltage signals 104, 106, 108 havingthe lowest voltage. However, other ones of the voltage signals 104, 106,108 also contribute to the error signal 118, but with less influence.

A particular desired value of the regulated output voltage 38 isdescribed above in conjunction with FIG. 1, and the same particulardesired value applies in the same way to the regulated output voltage122. The above described error signal 118, which is dominated by alowest one or more of the voltage signals 104, 106, 108, approximatelyachieves the particular desired value of the regulated output voltage122.

Certain elements of the circuit 90 can be within a single integratedcircuit. For example, in some arrangements, the current regulators 20,22, 24, the switches 104, 106, 108, the digital channel select circuit110, the amplifier 116, the capacitor 120, and some internal elements ofthe controllable DC-DC converter 12 (described more fully above inconjunction with FIG. 2) can be within the single integrated circuit.

In some alternate arrangements, the error amplifier 116 can be replacedby a comparator coupled to a digital integrator (or a counter) thatgenerates a weighted sum of the outputs from the comparator associatedwith closures of the switches 98, 100, 102. In other alternatearrangements, the error amplifier 116 can be replaced by a comparator,which generates an output signal that takes on a zero state (requestinga lower regulated output voltage 122) only when all of the currentregulators 20, 22, 24 are determined to be properly regulating.

In some alternate embodiments, the current regulators 20-24, which areshown to be coupled to the bottom (cathode) ends of the series connectedLED strings 14-18, respectively, can instead be at to top (anode) endsof the series connected LED strings 14-18, respectively. In theseembodiments, the input nodes 20 a-24 a are coupled to receive theregulated output voltage 38, and output nodes 20 b-24 b are coupled tothe anode ends of the series connected LED strings 14-18, respectively.Furthermore, in these embodiments, the switches 98-102 are coupled tothe output nodes 20 b-24 b, which become the voltage sense nodes inplace of the input nodes 20 a-24 a, and the non-inverting input of theerror amplifier 116 is coupled to receive a different reference voltage.

The circuit 90 has advantages over the prior art. For example, thecircuit 90 avoids the necessity for the above-described minimum selectcircuit, which can result in less integrated circuit die area.Furthermore, for embodiments in which the error amplifier 116 has anasymmetrical output drive capability as described above, a loop gain ofthe circuit 90 tends to change (e.g., drop) as more of the currentregulators 20, 22, 24 come into regulation, i.e., receive sufficientlyhigh voltage signals 92, 94, 96. The lower gain of the loop results in adrop of the error signal 118 as soon as any of the current regulators20, 22, 24 begin to regulate. For embodiments in which the controllableDC-DC converter 12 is a boost switching regulator (described more fullybelow in conjunction with FIG. 2), this tends to improve feedback loopstability and reduce overshoot and ringing that might occur during anyvoltage step, for example, at turn on of the circuit 90.

Referring now to FIG. 6, in which like elements of FIG. 1 are shownhaving like reference designations, an exemplary electronic circuit 130includes FETs 132, 134, 136, having drains coupled to cathode ends ofthe series connected LED strings 14, 16, 18, respectively. Sources ofthe FETs 132, 134, 136 are coupled to one end of resistors 138, 140,142, respectively, forming respective current sense nodes 150 a, 152 a,154 a, at which feedback signals 150, 152, 152 are generated.

The feedback signals 150, 152, 154 are coupled to inverting input nodesof amplifiers 144, 146, 148, respectively. A reference voltage signal156, for example, 0.2 volts, is coupled to the non-inverting input nodesof each one of the amplifiers 144, 146, 148. The resistors 138, 140, 142in combination with the respective amplifiers 144, 146, 148 are referredto herein as current sense circuits.

It should be appreciated that the feedback signal 150 a, 152 a, 154 aare representative of currents flowing through the resistors 138, 140,142, respectively. The feedback signals 150 a, 152 a, 154 a, aretherefore, not representative of voltages appearing at inputs of currentregulators (e.g., 20, 22, 24 of FIG. 1).

Amplifiers 144, 146, 148 are configured to generate voltage signals 162,164, 166, respectively. It will be recognized that the voltage signals162, 164, 166 are voltage signals that have voltage valuesrepresentative of currents flowing through the FETs 132, 134, 136,respectively. The voltage signals 162, 164, 166, are, therefore, alsonot representative of voltages appearing at inputs of current regulators(e.g., 20, 22, 24 of FIG. 1).

It is described above in conjunction with FIG. 1, that it is desirableto maintain a voltage at each one of the current regulators 20, 22, 24that is sufficiently high to allow proper operation of the currentregulators 20, 22, 24. One or more of the current regulators 20, 22, 24receive a lowest voltage. Accordingly, in the circuit 130, one or moreof the FETs receive a voltage signal 162, 164, 166 having a highestvoltage. The highest voltage is representative of one or more of thecurrent regulators 20, 22, 24 being turned on the most and being nearestto improper operation (i.e., shut off).

Accordingly, the voltage signals 162, 164, 166 are received by a maximumselect circuit 168, which is configured to select a highest one of thevoltage signals 162, 164, 166 and to pass through the highest one as thehighest voltage signal 169. Exemplary maximum select circuits aredescribed more fully below in conjunction with FIGS. 7 and 8.

An error amplifier 170 is coupled to receive the highest voltage signal169 at a non-inverting input node. The error amplifier 170 is alsocoupled to receive a reference voltage signal 172, for example, 2.5volts, at an inverting input node. The error amplifier 170 is configuredto generate an error signal 174 coupled to the error input node 12 b ofthe controllable DC-DC converter 12. The error amplifier 170 can have anoutput stage (not shown) with relatively equal source and sinkcapabilities. In some arrangements, the error amplifier 170 is atransconductance amplifier, which provides a current-type output.

The circuit 130 can include a capacitor 176 coupled to the output nodeof the error amplifier 170. The capacitor 176 can be comprised of aparallel combination of output capacitance of the error amplifier 170 inparallel with the input capacitance of the error node 12 b of thecontrollable DC-DC converter 12. However, in some other arrangements,the capacitor 176 can include another capacitor as well. In oneparticular arrangement, the capacitor 176 has a value of about onehundred picofarads. The capacitor 176 can provide a loop filter and canhave a value selected to stabilize a feedback control loop.

The controllable DC-DC converter 12 is coupled to receive the errorsignal 174 at the error node 12 b of the controllable DC-DC converter12. The controllable DC-DC converter 12 is also coupled to receive thepower supply voltage, Vps, at the input node 12 c and to generate aregulated output voltage 178 at the output node 12 a in response to theerror signal 174. It should be recognized that the regulated outputvoltage 178 can be the same as or similar to the regulated outputvoltage 38 of FIG. 1. However, since the error signal 174 is generatedin a different way than the error signal 34 of FIG. 1, the regulatedoutput voltage 178 need not be exactly the same as the regulated outputvoltage 38.

With this arrangement, the controllable DC-DC converter 12 is primarilycontrolled to keep all of the FETs 132, 134, 136 out of saturation,i.e., to keep a highest one of the voltage signals 162, 164, 166 below adesired value, while maintaining currents through the resistors 138,140, 142 at a predetermined value. Each one of the amplifier, FET, andresistor groups, for example the amplifier 144, the FET 132, and theresistor 138, operate as a current regulator, for which proper operationis maintained by controlling a highest one of the voltage signals 162,164, 166 by adjusting the regulated output voltage 178 to be just highenough (which can include a margin, for example, a one volt margin).

A desired largest error signal 174 achieves linear operation of the FET132, 134, 236 associated with the series connected LED string 14, 16, 18having the largest voltage drop. In one particular embodiment, thedesired largest error signal 174 is four volts or less, in accordancewith an amplifier 144, 146, 148 capable of generating an output signal162, 164, 166 of five volts or less.

A particular desired value of the regulated output voltage 38 isdescribed above in conjunction with FIG. 1, and the same particulardesired value applies in a similar way to the regulated output voltage178. The above described error signal 174 approximately achieves theparticular desired value of the regulated output voltage 178.

Certain elements of the circuit 130 can be within a single integratedcircuit. For example, in some arrangements, the FETs 132, 134, 136, theresistors 138, 140, 142, the amplifiers 144, 146, 148, the maximumselect circuit 168, the error amplifier 170, the capacitor 176, and someinternal elements of the controllable DC-DC converter 12 (described morefully above in conjunction with FIG. 2) can be within the singleintegrated circuit.

In some alternate embodiments, the FETs 132-136, the resistors 138-142,and the amplifiers 144-148, which are shown to be at the bottom ends ofthe series connected LED strings 14-18, respectively, can instead be atthe top ends of the series connected LED strings 14-18, respectively.

The circuit 130 has advantages over the prior art. In operation, thecircuit 130 is able to regulate the controllable DC-DC converter 12 toachieve a regulated voltage 178 that assures that none of the FETs 132,134, 136 go into current starvation, i.e., they can regulate current asdesired. In contrast, use of the above-described minimum select circuitin prior art results in a desired regulated output voltage 178 able toprovide enough voltage to associated current regulators. As describedabove, with the prior art arrangement, a voltage margin, e.g., one volt,is often used to assure than none of the associated current regulatorswill be current starved. Thus, the prior art tends to waste some powerin the current regulators by way of the voltage margin, whereas thecircuit 130 can operate without a margin or with a smaller margin.

Referring now to FIG. 7, a circuit 200 can be used as the maximum selectcircuit 168 of FIG. 6. The circuit 200 includes three input nodes 202,204, 206 coupled to respective cathode ends of diodes 208, 210, 212.Anode ends of the diodes 208, 210, 212 are coupled together to an inputnode of a current regulator 214 and to an output node 216 of the circuit200.

It will be appreciated that an output signal VMAX appearing at theoutput node 216 is a largest one of input signals appearing at the inputnodes 202, 204, 206.

Referring now to FIG. 8, another circuit 230 can be used as the maximumselect circuit 168 of FIG. 6. The circuit 230 includes three input nodes232, 234, 236 coupled to gates of respective FETs 238, 240, 242. Drainsof the FETs 238, 240, 242 are coupled together and to a source of a FET244. Sources of the FETs 238, 240, 242 are coupled together and to aninput node of a current regulator 250. A gate of the FET 244 is coupledto a gate of a FET 246 and also to the source of the FET 244. A sourceof the FET 246 is coupled to an output node 252 of the circuit 230. Theoutput node 252 is coupled to a gate and a drain of a FET 248. A sourceof the FET 248 is coupled to the input node of the current regulator250.

It will be appreciated that an output signal VMAX appearing at theoutput node 252 is a largest one of input signals appearing at the inputnodes 238, 240, 242.

Referring now to FIG. 9, in which like elements of FIGS. 1 and 5 areshown having like reference designations, an exemplary electroniccircuit 270 includes the switches 98, 100, 102, coupled to receivevoltage signals 272, 274, 276 respectively. The voltage signals 272,274, 276 can be the same as or similar to the voltage signals 26, 28,30, respectively, of FIG. 1 or the voltage signals 92, 94, 96 of FIG. 5.The switches 98, 100, 102 are also coupled to receive a control signal298 generated by a digital channel select module 296, which causes theswitches 98, 100, 102 to open and close, but in a different way than thedigital channel select module 110 of FIG. 5. Operation of the digitalchannel select module 296 is described more fully below.

An error amplifier 290 is coupled to receive a composite signal 286 atan inverting input node, to receive the reference voltage 115, forexample, 0.5 volts, at a non-inverting input node, and configured togenerate an error signal 300. Unlike the error amplifier 116 of FIG. 5,an output stage (not shown) of the error amplifier 290 can be configuredto provide currents with generally symmetrical drive capability in bothdirections. In some arrangements, the error amplifier 290 is atransconductance amplifier, which provides a current-type output.

The circuit 270 can include a capacitor 302 coupled to the output nodeof the error amplifier 116. The capacitor 302 can be comprised of aparallel combination of output capacitance of the error amplifier 290 inparallel with the input capacitance of the error node 12 b of thecontrollable DC-DC converter 12. However, in some other arrangements,the capacitor 302 can include another capacitor as well. In oneparticular arrangement, the capacitor 302 has a value of about onehundred picofarads. The capacitor 302 can provide a loop filter and canhave a value selected to stabilize a feedback control loop.

The controllable DC-DC converter 12 is coupled to receive the errorsignal 300 at the error node 12 b of the controllable DC-DC converter12. The controllable DC-DC converter 12 is also coupled to receive thepower supply voltage, Vps, at the input node 12 c and to generate aregulated output voltage 304 at the output node 12 a in response to theerror signal 300. It should be recognized that the regulated outputvoltage 304 can be the same as or similar to the regulated outputvoltage 38 of FIG. 1 or the regulated output voltage 122 of FIG. 5.However, since the error signal 300 is generated in a different way thanthe error signal 34 of FIG. 1 and the error signal 118 of FIG. 5, theregulated output voltage 304 need not be exactly the same as theregulated output voltages 38, 122.

The electronic circuit 270 can also include a comparator 292, having oneinput node coupled to receive the reference voltage 115 and anotherinput node coupled to receive the composite signal 286. The comparator294 is configured to generate a comparison signal 294, which is receivedby the digital channel select module 296.

In operation, the digital channel select module 296 selects particularchannels, one at a time, and closes the switches 98, 100, 102, one at atime, accordingly. The digital channel select module 296 keeps theselected switch closed for at least some predetermined minimum time, forexample, one microsecond. Both the error amplifier 290 and thecomparator 294 are coupled to receive one of the voltage signals 272,274, 276 in accordance with the selected one of the switches 98, 100,102. The selected one of the switches 98, 100, 102 remains closed untilsuch time as the associated one of the current regulators 20, 22, 24achieves proper current regulation, i.e., until its associated voltagesignal 272, 274, 276 is sufficiently high. When the associated one ofthe current regulators 20, 22, 24 achieves proper current regulation,then the digital channel select module 296 switches to a next channel,i.e., selects a different one of the switches 98, 100, 102 to close.Operation of the digital channel select module 296 continues in thisway, continuously sequencing through the switches 98, 100, 102.

With this arrangement, due in-part to averaging provided by thecapacitor 302, the controllable DC-DC converter 12 is primarilycontrolled by a lowest one or more of the voltage signals 272, 274, 276,which tends to receive a longest closure of an associated one of theswitches 98, 100, 102, and other ones of the voltage signals 272, 274,276 can have less influence. Thus, a voltage signal 272, 274, 276 thatwould otherwise be too low to provide proper operation of an associatedone of the current regulators 20, 22, 24 will result in an increase inthe error signal 300, tending to raise the regulated output voltage 304of the controllable DC-DC converter 12.

With this arrangement, the controllable DC-DC converter 12 is controlledpredominantly by one or more of the voltage signals 272, 274, 276 havingthe lowest voltage. However, other ones of the voltage signals 272, 274,276 also contribute to the error signal 300, but with less influence.

A particular desired value of the regulated output voltage 38 isdescribed above in conjunction with FIG. 1, and the same particulardesired value applies in the same way to the regulated output voltage302. The above described error signal 300, which is dominated by alowest one or more of the voltage signals 272, 274, 276, approximatelyachieves the particular desired value of the regulated output voltage304.

Certain elements of the circuit 270 can be within a single integratedcircuit. For example, in some arrangements, the current regulators 20,22, 24, the switches 104, 106, 108, the digital channel select circuit296, the error amplifier 290, the comparator 292, the capacitor 302, andsome internal elements of the controllable DC-DC converter 12 (describedmore fully above in conjunction with FIG. 2) can be within the singleintegrated circuit.

In some alternate arrangements, the error amplifier 290 can be replacedby a comparator coupled to a digital integrator (or a counter) thatgenerates a weighted sum of the outputs from the comparator associatedwith closures of the switches 98, 100, 102. In other alternatearrangements, the error amplifier 290 can be replaced by a comparator,which generates an output signal that takes on a zero state (requestinga lower regulated output voltage 304) only when all of the currentregulators 20, 22, 24 are determined to be properly regulating.

In some alternate embodiments, the current regulators 20-24, which areshown to be coupled to the bottom (cathode) ends of the series connectedLED strings 14-18, respectively, can instead be at to top (anode) endsof the series connected LED strings 14-18, respectively. In theseembodiments, the input nodes 20 a-24 a are coupled to receive theregulated output voltage 38, and output nodes 20 b-24 b are coupled tothe anode ends of the series connected LED strings 14-18, respectively.Furthermore, in these embodiments, the switches 98-102 are coupled tothe output nodes 20 b-24 b, which become the voltage sense nodes inplace of the input nodes 20 a-24 a, and the non-inverting input of theerror amplifier 290 is coupled to receive a different reference voltage.

The circuit 270 has advantages over the prior art. For example, thecircuit 270 avoids the necessity for the above-described minimum selectcircuit, which can result in less integrated circuit die area.

The arrangements of FIGS. 1, 3, 4, 5, 6, 7, 8, and 9 are indicative ofthree series connected strings of light emitting diodes. However, itwill be appreciated that other circuits can be expanded or contracted toaccommodate more than three or fewer than three series connected stringsof light emitting diodes, including one series connected string of lightemitting diodes.

As described above, the arrangements of FIGS. 1, 4, 5, 6, and 9 show theregulated output voltage of the controllable DC-DC converter 12 coupledto the anode ends of the series connected LED strings 14, 16, 18, andcurrent regulators (e.g., 20, 22, 24, FIG. 1) or other components (e.g.,132, 134, 136, FIG. 6) coupled between the cathode ends of the seriesconnected LED strings 14, 16, 18 and ground. It will be appreciated thatother similar arrangements are also possible, for which the regulatedoutput voltage of the controllable DC-DC converter 12 is instead coupledto the current regulators and the current regulators are in turn coupledto the anode ends of the series connected LED strings 14, 16, 18, whichare coupled at their cathode ends to ground. Furthermore, still otherarrangements are possible for which the regulated output voltage of thecontrollable DC-DC converter 12 is a negative voltage.

All references cited herein are hereby incorporated herein by referencein their entirety. Having described preferred embodiments of theinvention, it will now become apparent to one of ordinary skill in theart that other embodiments incorporating their concepts may be used.

It is felt therefore that these embodiments should not be limited todisclosed embodiments, but rather should be limited only by the spiritand scope of the appended claims.

What is claimed is:
 1. An electronic circuit for driving a plurality ofseries connected light emitting diode strings with a controllable DC-DCconverter, the electronic circuit comprising: a plurality of currentregulators, each having a respective input node and a respective outputnode, the input node or the output node coupled to an end of arespective one of the plurality of series connected light emitting diodestrings, wherein each current regulator is configured to pass arespective predetermined current through the respective one of theplurality of series connected light emitting diode strings to which itis coupled; and a plurality of error amplifiers, each having arespective input node and a respective output node, wherein each one ofthe plurality of input nodes of the plurality of error amplifiers iscoupled to the input node or the output node of a respective one of theplurality of current regulators, wherein the output nodes of theplurality of error amplifiers are coupled to a junction node, whereinthe plurality of error amplifiers is configured to generate an errorsignal at the junction node.
 2. The electronic circuit of claim 1,wherein each one of the plurality of error amplifiers is configured tobe able to sink a different amount of current than it is able to source.3. The electronic circuit of claim 1, wherein the controllable DC-DCconverter is coupled to receive the error signal, wherein thecontrollable DC-DC converter comprises an input node configured toreceive a voltage and an output node at which regulated output voltageis generated by the controllable DC-DC converter, wherein the errorsignal is configured to control the regulated output voltage.
 4. Theelectronic circuit of claim 3, wherein the controllable DC-DC convertercomprises a switching regulator.
 5. The electronic circuit of claim 4,wherein each one of the plurality of error amplifiers is configured tobe able to sink a different amount of current than it is able to source.6. The electronic circuit of claim 1, wherein the plurality of erroramplifiers comprises a plurality of transconductance amplifiers.
 7. Theelectronic circuit of claim 6, further comprising a capacitor coupled tothe junction node forming a loop filter.
 8. A method of driving aplurality of series connected light emitting diode strings with acontrollable DC-DC converter, the method comprising: attempting to passa respective predetermined current through each one of the plurality ofseries connected light emitting diode strings, resulting in a respectivevoltage appearing at an end of each one of the plurality of seriesconnected light emitting diode strings; generating respectiveintermediate signals representative each one of the voltages; andsumming the intermediate signals to generate an error signal to controlthe DC-DC converter.
 9. The method of claim 8, wherein the generatingthe respective intermediate signals comprises: receiving the respectivevoltages with a plurality of error amplifiers, wherein each one of theplurality of error amplifiers is configured to be able to sink adifferent amount of current than it is able to source.
 10. The method ofclaim 9, wherein the plurality of error amplifiers comprises a pluralityof transconductance amplifiers.
 11. The method of claim 10, furthercomprising filtering the error signal to form a loop filter.
 12. Themethod of claim 8, wherein the controllable DC-DC converter is coupledto receive the error signal, wherein the controllable DC-DC convertercomprises an input node configured to receive a voltage and an outputnode at which regulated output voltage is generated by the controllableDC-DC converter, wherein the error signal is configured to control theregulated output voltage.
 13. The method of claim 12, wherein thecontrollable DC-DC converter comprises a switching regulator.
 14. Themethod of claim 13, wherein the generating the respective intermediatesignals comprises: receiving the respective voltages with a plurality oferror amplifiers, wherein each one of the plurality of error amplifiersis configured to be able to sink a different amount of current than itis able to source.
 15. The method of claim 14, wherein the plurality oferror amplifiers comprises a plurality of transconductance amplifiers.16. The method of claim 15, further comprising filtering the errorsignal to form a loop filter.
 17. The method of claim 8, wherein thegenerating the respective intermediate signals comprises: receiving therespective voltages with a plurality of error amplifiers.
 18. The methodof claim 17, wherein the plurality of error amplifiers comprises aplurality of transconductance amplifiers.
 19. The method of claim 18,further comprising filtering the error signal to form a loop filter.